Theoretical study on decomposition pathways and reaction rate constants of C4F7N with O atom

Abstract

C4F7N is one of the promising candidates to replace SF6 as insulating medium in power equipment. Meanwhile, the effect of an O atom on C4F7N decomposition is still unclear but it greatly influences the decomposition mechanism and insulating properties of C4F7N. In this paper, the B3LYP method, in conjunction with 6-311G(d,p) basis set, is used to study the decomposition mechanism of C4F7N with an O atom. The molecular structures of reactants, products and transition states (TS) in the C4F7N decomposition pathway, which consists of 25 reactions, are optimized. The potential energy surface of each reaction and the vibrational frequencies of each molecule or TS are studied with same method as the prerequisite for rate constant calculations using transition state theory. The results show that the CN bond breaks with the highest energy of 321.91 kcal mol−1, so reaction (R21) (C4F7NO → C4F7O + N) plays a less important role than (R2) (C4F7NO → C3F4NO + CF3), (R5) (C4F7NO → C3F7O + CN), (R11) (C4F7NO → TS1 → C3F5N + CF2O) and (R15) (C4F7NO → TS2 → C3F3NO + CF4) in the decomposition of C4F7NO. CF2O and CF4, characteristic decomposition products of C4F7N + O which have negative impacts on the insulating gas, are generated in reactions (R11) and (R15) with a higher rate constant at 1500 K above, respectively. CF3 and F3C–O are important in C4F7N with O decomposition because of the high overall-generation rate constants. This work is expected to provide a theoretical basis to evaluate the insulation performance of C4F7N-insulated power equipment and develop novel gas sensors for insulating condition monitoring.